Propeller spinners comprise the central nose portion of a propeller, providing an aerodynamically shaped cap which covers and protects the attaching assembly which couples the propeller to a drive shaft. Propeller spinners, including those made from polyester composite materials, are known in the art.
During operation, various foreign objects, such as birds or dislodged ice, may contact an engine by striking the propeller spinner. Generally, polyester composite propeller spinners, utilizing a polyester resin and about 35% by weight of a glass fiber filler, were thought to have sufficient impact strength to resist damage from such object strikes. However, damage has occurred in some propeller spinners which were struck by ice dislodged from the propeller blades. After an evaluation of the impact damage, it was determined that the standard impact strength of 3.32 Kg-m (24 ft-lbs), measured using a drop impact test, is inadequate to assure safe operation of a polyester composite propeller spinner.
Several changes in the resin or fiber filler can be made to increase impact strength. For instance, the filler can comprise glass fibers of greater length. Generally, the use of longer fibers embedded in a matrix resin promotes increased impact strength. However, the length of fiber is generally limited to the maximum length which the glass fiber chopping equipment can provide. Also, handling characteristics and workability are adversely affected as the fibers get longer. Maintaining 35% by weight of glass fiber while increasing the fiber length to the maximum provided by a typical chopping machine, produces a 17% improvement in impact strength, from the base impact strength of 3.32 Kg-m (24 ft-lbs) to a final impact strength of 3.87 Kg-m (28 ft-lbs). However, processing time is increased due to the adverse effects on workability.
Additives are also known in the art for improving the impact strength of polyester composites. In particular, rubbery polymers such as carboxyl terminated butadiene-styrene copolymer may be added to a polyester resin to improve impact strength and toughness. However, such rubbery polymers have the additional effect of flexibilizing the resin matrix, decreasing the composite stiffness and lowering the heat resistance. These changes in the structural properties of a propeller spinner are undesirable.
Another additive known in the art for improving impact strength is a vinyl terminated liquid reactive compound, which improves the impact strength of a polyester composite without detrimentally flexibilizing the resin matrix or changing other structural properties of the resin. The addition of the vinyl compound to a polyester resin using standard length fibers adds 1.10 Kg-m (8 ft-lbs) to the impact strength of a polyester composite propeller spinner, for a total impact strength of 4.42 Kg-m (32 ft-lbs). While a 33% improvement over the standard propeller spinner, it does not provide the impact strength required to prevent ice impact damage.
A change in the type of fiber, for example from glass to carbon, could add significantly to impact strength. However, such carbon fibers are on the order of 12 times as expensive as glass fibers and are not considered an economically acceptable alternative.